Treatment of sour hydrocarbon distillate

ABSTRACT

An apparatus and process is described whereby a sour hydrocarbon distillate stream is treated in two oxidation zones within the same reaction vessel to oxidize mercaptans. The initial treatment is in the presence of a fiber bundle and the subsequent treatment over a bed of supported oxidation catalyst.

BACKGROUND OF THE INVENTION

This invention relates to the treatment of sour hydrocarbon distillateto remove mercaptan compounds by contacting the distillate with analkaline solution.

More particularly, this invention relates to the removal of suchmercaptans by a two-stage oxidation contact wherein the first oxidationof the mercaptan compounds occurs while the hydrocarbon and the alkalinesolution are in contact with a bundle of a plurality of elongatedfibers.

The art relating to the treatment of sour distillate hydrocarbons iswell developed and the processes therefor are the subject matter of manypatents. For example, U.S. Pat. Nos. 3,758,404, 3,977,829 and 3,992,156describe mass transfer apparatus and processes involving the use offiber bundles which are particularly suited for such processes. It is animprovement over the inventions described in such patents and othersthat this invention is made for the treatment of hydrocarbon distillatescontaminated with mercaptan compounds requiring more treatment residencetime than are practically possible in a single phase bundle treatingprocess as described in the above mentioned patents in order toaccomplish sufficient removal. While satisfactory treatment could beaccomplished in a series of such treatment apparatus, it then becomesvery expensive and uneconomic. Thus the instant invention is animprovement over such described methods.

The treatment process of this invention will be recognized by thoseskilled in the art to be particularly suitable for the treatment toeffect the catalytic oxidation of the offensive mercaptans contained insour hydrocarbon distillate such as for example gasoline, includingnatural straight run and cracked gasolines, naphtha, kerosene, jetfuels, fuel oil and the like.

Commonly used processes for treating sour hydrocarbon distillates entailtreating the distillate in contact with a metal phthalocyanine catalystdispersed in an aqueous caustic solution to yield a doctor sweetproduct. The sour distillate and the catalyst-containing aqueous alkalimetal hydroxide solution provide a liquid-liquid system whereinmercaptans are converted to disulfides in the presence of an oxidizingagent --usually an oxygen contained gas dissolved in the hydrocarbonbeing treated. Sour hydrocarbon distillates containing more difficultyoxidizable mercaptans are more effectively treated in contact with ametal phthalocyanine catalyst deposited on a high surface areaadsorptive support--usually a metal phthalocyanine on an activatedcharcoal. The distillate is treated in contact with the supported metalphthalocyanine catalyst at oxidation conditions in the presence of anaqueous alkaline solution. One such process is described in U.S. Pat.No. 2,988,500. The oxidizing agent is more often air admixed with thedistillate to be treated, and the aqueous alkaline agent is most oftenan aqueous alkali metal hydroxide, or caustic solution chargedcontinuously to the process or intermittently as required to maintainthe catalyst in a caustic-wetted state.

The prior art recognizes that there are limitations on the ability totreat a sour hydrocarbon distillate with a catalytic compositeconsisting of a metal phthalocyanine disposed on a support material suchas the relatively short catalyst life plugging of the catalyst bed, andthe required utilization of aqueous-phase alkaline reagents. Variousimprovements have been developed to further enhance the sweeteningability, including the use of certain additives in the distillatetreating process, for instance, a method comprising contacting thedistillate at oxidation conditions with a supported metal chelatemercaptan oxidation catalyst and anhydrous ammonia in the absence of anaqueous phase. (U.S. Pat. No. 4,502,949).

It has now been surprisingly discovered that the difficulty oxidizablemercaptan compounds are economically and effectively oxidized andremoved to meet mercaptan content specifications of distillatehydrocarbons by the invention as described hereinafter.

SUMMARY OF THE INVENTION

This invention involves an improvement over the treating processemploying the equipment and apparatus described in the above mentionedpatents whereby the hydrocarbon containing the mercaptan compounds to beoxidized are contacted with an aqueous alkali metal hydroxide solution,an oxidizing agent, preferably air, and a soluble metal phthalocyaninecatalyst while in contact with a bundle of elongated fibers contained ina conduit. Upon disengagement from the fiber bundle, the hydrocarbon andthe aqueous alkali metal hydroxide solution separate by gravity in aseparation zone of the vessel where the aqueous solution becomes a lowerphase and the hydrocarbon containing the oxidized mercaptan compounds,which remains as an upper oil phase. This improvement involves thenpassing the hydrocarbon phase, usually upwardly, through a packedcatalyst bed, usually in annular arrangement about the conduitcontaining the bundle fibers. There, the remaining difficultlyoxidizable mercaptan compounds are further oxidized in contact with asupported metal oxidation catalyst, preferably on a carbon support,while being contacted in countercurrent flow with additional aqueousalkali metal hydroxide solution flowing through the catalyst bed. Thus,by so using the second treatment stage with the fixed supportedcatalyst, longer residence times are possible without the need for thecapital expenditure for a second vessel to hold the supported catalystbed.

The additional residence time in contact with the solid catalyst bedafter the more easily oxidized lower mercaptans are oxidized in contactwith the fibers provides highly efficient and economic treatment ofdifferent sour hydrocarbon distillates containing high levels ofmercaptans.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in schematic form, the apparatus used in the practice ofthis invention.

FIG. 2 shows in schematic form an embodiment of this invention employinga horizontally oriented catalyst bed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, FIG. 1 shows a preferred embodiment withconduit 10 having in it a bundle of elongated fibers 12 preferablymetallic fibers secured in the conduit 10 for a portion of its length.These fibers 12 are secured at the upstream end in conduit 10, and fedthrough dispensing means, or distribution grid 14 with aqueous alkalimetal hydroxide from tube 16. The hydrocarbon distillate to be treatedenters through line 18 fed by charge stock from line 19 which is mixedwith oxygen, preferably in form of air, through sparger 20 from intakeline 21. Sufficient oxygen will normally be dissolved to oxidize allmercaptans in the hydrocarbon.

At the downstream end of the conduit 10 is a gravity separator 24 intowhich the downstream end of the fibers 12 extend. This separator 24 ispreferably integrated with the Vessel V enclosing the conduit 10.

In operation of the apparatus of FIG. 1 an aqueous alkali metalhydroxide, or caustic, solution is introduced through the tube 16 anddistribution means 14 onto the fibers 12. A sour hydrocarbon stream,such as a hydrocarbon distillate, containing mercaptans alone ormercaptans and acidic impurities is introduced into the conduit 10through the inlet pipe 18. The fibers 12 will be wetted by the aqueouscaustic solution in preference to the hydrocarbon mixture. The aqueouscaustic solution will form a film on the fibers 12 which will be draggeddownstream through the conduit 10 by the passage of the hydrocarbondistillate through the same conduit. Both liquids will be dischargedinto the separation zone 26 of the separator 24. The volume of thehydrocarbon will be greater because the aqueous caustic passes throughthe system at a lower volumetric flow rate than the hydrocarbon. Duringthe relative movement of the hydrocarbon with respect to the aqueouscaustic film on the fibers, a new interfacial boundary between thehydrocarbon distillate and the aqueous caustic solution is continuouslybeing formed, and as a result fresh aqueous caustic solution is broughtin contact with this surface and allowed to react with the mercaptans inthe hydrocarbon. While in contact with the fiber bundle 12, acidicimpurities commonly found in a hydrocarbon charge stock, phenolics,naphthenic acid and other organic acids are removed from the hydrocarbondistillate.

In the separation zone 26, the aqueous caustic solution will collect inthe lower portion as it is heavier than, and immiscible in, thehydrocarbon. The interface 30 within the separator vessel 24 is normallykept at a level above the bottom of the downstream end of the fibers 12so that the aqueous caustic film can be collected directly in the bottomof the separator 24 without it being dispersed into the hydrocarbon.Thus separated, the hydrocarbon no longer contains phenolate ornaphthenate impurities which often cause plugging in a packed bed. Notonly does this increase oxidation efficiency, but reduces maintenancecosts as well.

The conduit 10 and the fiber bundle 12 enclosed therein are designed,and sized, to have a length and diameter to preferrably allow a 10centimeter per second flow rate and approximately a 1 minute residencetime for contact. The aforementioned parameters are preferred and thespeed at which the liquids proceed from one end to the other of thebundle 12 can preferably vary from about 2 to about 20 centimeters persecond and the residence time from about 30 seconds to about 5 minutes.Longer residence times in contact with the fiber bundle would cause theconduit 10 and the fibers 12 to be of an inordinate length but, becauseof this invention such lengthy file bundles are unnecessary.

Concurrently with the introduction of hydrocarbon having anoxygen-containing gas, preferably air, dissolved therein, there isintroduced through the distribution grid 14 from line 16 an aqueousalkali metal hydroxide, preferably sodium hydroxide, having aconcentration of from about 5% to about 50% by weight alkali hydroxidewith about 10% to 20% by weight alkali hydroxide being preferred. Theamount of aqueous alkali metal hydroxide introduced through thedistribution grid 14 is such that the volumetric ratio of hydrocarbondistillate to aqueous caustic is from about 2:1 to about 20:1 with theratio of from 3:1 to 7:1 being preferred and about 5:1 being especiallypreferred.

In addition the alkali metal hydroxide solution also includes anoxidation catalyst, the most prevalent and well known catalyst being asoluble phthalocyanine catalyst.

The phthalocyanine catalyst is both very active and highly stable.Because of its high activity, the catalyst is used in smallconcentrations. These may range from 5 to 500 and preferably 10 to 100parts per million by weight of the alkaline solution, although lower orhigher concentrations may be used in some cases. The use of higherconcentrations are unnecessary in most cases but may be used if desired.Because of its high stability, the catalyst is used for long periods oftime.

Any suitable alkaline solution is utilized in the process and comprisesparticularly sodium hydroxide (caustic), potassium hydroxide, etc. Thealkaline solution generally is utilized as an aqueous solution of fromabout 5% to about 50% by weight concentration, more preferably about 10%to 20% by weight concentration. When desired, solutizers, solubilizingagents, and the like are employed including, for example, alcohols,particularly methanol or ethanol, phenols, cresols, butyric acid,naphthenic acid and so forth, in order to increase the contact and/orreaction of the sulfur compounds with the alkaline reagent. In somecases the hydrocarbon distillate contains solutizing agents insufficient concentration to serve this purpose; otherwise they may beintroduced from an extraneous source.

Any suitable phthalocyanine catalyst meeting the requirements of highactivity and stability during use may be employed in the presentinvention. Particularly preferred metal phthalocyanines comprise cobaltphthalocyanine and vanadium phthalocyanine. The metal phthalocyanine ingeneral, is not readily soluble in aqueous solutions and therefore, forimproved operation is preferably utilized as a derivative thereof.Particularly preferred derivatives are the sulfonated and carboxylatedderivatives, and more particularly the disulfonated derivatives. Thus, apreferred phthalocyanine catalyst comprises cobalt phthalocyaninedisulfonate. Another preferred catalyst comprises vanadiumphthalocyanine disulfonate.

The aqueous caustic separates from the partially treated hydrocarbon inthe separation zone 26 with the aqueous caustic collecting at the bottomof the vessel with the partially treated hydrocarbon at the top of theseparator 24 as the top layer outside of the conduit 10 containing thefibers 12 where the hydrocarbon becomes disengaged from the fibers 12.In annular arrangement about the conduit 10 in the vessel V is acatalyst bed C.

In addition to the oxidation catalyst dispersed in the alkali metalhydroxide solution, an oxidation catalyst, preferably a similarphthalocyanine catalyst, is composited with a suitable support. Thesupport should be insoluble in, or substantially unaffected by thecaustic solution and hydrocarbons under the conditions prevailing inthis subsequent treating zone 38 of the catalyst bed C. Activated carbonis particularly preferred because of its high adsorptivity and stabilityunder these conditions. Other carbon carriers include coke, charcoalwhich may be obtained from any suitable source including bone char, woodcharcoal, charcoal made from cocoa-nut or other nut shells, fruit pits,etc. The choice of the support will be made with reference to itsadsorptive or spacing properties and its stability in the alkalinereagent solution and hydrocarbons at the conditions prevailing in thetreating zone.

The composite of phthalocyanine and support may be prepared in anysuitable manner. In one method the support may be formed into particlesof uniform or irregular size and shape, including spheres, prills,pellets rings, saddles, flakes, etc. and then is intimately contactedwith the solution of the phthalocyanine catalyst. An aqueous, oralkaline, solution of the phthalocyanine catalyst is prepared and, in apreferred embodiment, the support particles are soaked, dipped,suspended, or immersed in the solution. In another method, the solutionmay be sprayed onto, poured over or otherwise contacted with thesupport. Excess solution may be removed in any suitable manner, and thesupport containing the catalyst allowed to dry at room temperature, inan oven, or by means of hot gases passed thereover, or in any othersuitable manner.

In general it is preferred to composite as much catalyst with thesupport as will form a stable composite although a lesser amount may beso deposited, if desired. In a typical preparation, 1% by weight, ofcobalt phthalocyanine sulfonate is composited with activated carbon bysoaking granules of the carbon in a solution of the phthalocyaninecatalyst. In another method, the carrier may be deposited in the vesselV to form the bed C and the phthalocyanine catalyst solution passedtherethrough with subsequent drying in order to form the catalystcomposite in situ. If desired, the solution may be recycled one or moretimes in order to prepare the desired composite. In still anotherembodiment the carrier may be placed in the vessel V and the vessel Vfilled with a solution of the catalyst, thereby forming the composite insitu.

The hydrocarbon passes through the catalyst bed C which is designed tohave a diameter and length to allow a residence time of from about 5minutes to about 60 minutes with the preferable residence time beingabout from 15 to about 25 minutes, most preferably about 20 minutes. Acatalyst bed C is supported by a restrainer means such as a screen 40 inthe vessel V. As the hydrocarbon being treated is disengaged from thefibers 12 and moves up through the catalyst bed C it is contacted byfresh alkali metal hydroxide in the treating zone 38 being introducedinto the bed C through distributors 42 placed within the catalyst bed C.While placement for the distributors 42 is arbitrary, it should betowards the top of the catalyst bed, preferably in the top 25% of thebed, to allow the caustic being introduced at a ratio of up to 1 part byvolume per part 5 parts by volume of the hydrocarbon distillate,containing dissolved oxidation gases as hereinbefore discussed tooxidize the mercaptan compounds in the hydrocarbon. The aqueous causticsolution exits the treating zone 38 through the restraining means 40into the separator 24 wherein it then exits the separation zone 26through line 50 to pump 52 where it is moved through line 54 to berecirculated for reuse through line 56 into line 16 through thedistributors 14 on to the bundle 12. Impurities are removed from theaqueous caustic stream from line 54 through purge line 58 and valve 60which operates in response to an interface level controller 62monitoring the aqueous liquid level of the separation zone 26 andremoves excess liquid in response to signals from the level controller62.

The product hydrocarbon containing organic disulfides oxidation productis removed from the catalyst bed C and the reaction vessel V throughproduct line 64 from the collection means 66, which can be anycollection means well known in the art. Excess oxidant, usually oxygengas, invented from the hydrocarbon distillate product through anappropriate vent 67. This vent is shown on vessell V but it is to beunderstood that it could be from a separately located flash tank. Also,shown on FIG. 1 are access flanges 68 and 70 which are used formaintenance of the vessel V and nozzles 72 and 74, arbitrarily placed ina convenient location for the charging of catalyst to the vessel V.

FIG. 2 is a schematic which shows the bed C disposed in a horizontalconfiguration about the conduit 110. The foregoing description isequally applicable to describe the embodiment of FIG. 2 and thereforethe last two digits of the numbers identifying like elements in FIG. 1are identical with the same element in the description thereof. Ratherthan repeat the foregoing discussion of the vertically arranged reactor,the discussion is incorporated herein by reference, with the numberingchanges mentioned above.

The process of this invention to oxidize mercaptans contained in thehydrocarbon feed described above takes place in contact with the fiberbundle 12 and 112 in the conduit 10 and 110 at a temperature of betweenabout 60° F. (16° C.) and about 200° F. (93° C.), preferably from about90° F. (32° C.) and about 150° F. (66° C.) at a pressure of from about2.5 to about 15 atmospheres, preferably from about 3 to about 10atmospheres. The foregoing temperatures and pressures allow theseparation of excess oxygen and any inert dissolved gases from thehydrocarbon through vent 67 or 167 without hydrocarbon entrainment orloss with the vented gas.

The same temperatures and pressures of course prevail as the hydrocarbondistillate courses upwardly through the catalyst bed C, in bothembodiments, nests upon a restraining means 40 and 140 positioned abovethe average location of the interface 30 and 130 of the hydrocarbon andaqueous fluids and preferably at a distance of about twice the diameterof the conduit 10 and 110. Referring to FIG. 2, the diameter of theconduit 110 is shown as D2 and the vessel V as D1. The restraining means140 is placed at a distance of about twice D2.

Thus, from the above discussion the great advantages of this inventionare apparent. The fiber bundle oxidation step acts to pre-treat thehydrocarbon stream by oxidizing the lower mercaptans and removing thephenolates and naphthenates which have bed-plugging potential and thepacked catalyst bed provides additional catalyst and residence time tooxidize the more difficultly oxidized mercaptans. In view of theforegoing, those of ordinary skill in the art will perceive, and canpractice many variations of the foregoing invention without departingfrom the scope and purposes of the claims hereof.

What is claimed is:
 1. In the process of oxidizing mercaptan compoundscontained in hydrocarbon distillates, comprising the steps of contactingthe distillate in the presence of a fiber bundle with a gaseous oxygenoxidant and an aqueous alkali metal hydroxide solution containing anoxidation catalyst for a period of time sufficient to oxidize at least aportion of the mercaptan compounds in the hydrocarbon distillate andseparating the aqueous alkali metal hydroxide from the treatedhydrocarbon distillate, the improvement which comprises:disengaging thehydrocarbon distillate and oxidant gas from the presence of the fiberbundle; contacting the hydrocarbon distillate and oxidant gas with a bedof supported oxidation catalyst in the presence of an aqueous alkalimetal hydroxide solution having a content of from about 5 wt % to about50 wt % of an alkali metal hydroxide for a residence time of from about5 minutes to about 60 minutes; and recovering the product hydrocarbondistillate from such catalyst zone.
 2. The process of claim 1 whereinthe supported oxidation catalyst is a metal phthalocyanine supported ona carbon support.
 3. The process of claim 2 wherein the catalyst iscobalt phthalocyanine disulfonate.
 4. The process of claim 3 wherein thesupport is an activated carbon.
 5. The process of claim 1 wherein thealkali metal hydroxide in sodium hydroxide in a concentration from about10% to about 20% by weight and the residence time is from about 15 toabout 25 minutes.
 6. An improved process for treating sour hydrocarbondistillate having mercaptan compounds therein, wherein said sourhydrocarbon distillate may also have acidic impurities therein,comprising the steps of:contacting said hydrocarbon distillate in thepresence of a fiber bundle with a gaseous oxygen oxidant and a firstaqueous alkali metal hydroxide solution having an oxidation catalysttherein at a temperature and pressure and for a period of timesufficient to oxidize at least a portion of said mercaptan compounds andto remove at least a portion of said acidic impurities from saidhydrocarbon distillate; separating said first aqueous alkali metalhydroxide solution from the treated hydrocarbon distillate; contactingsaid treated hydrocarbon distillate and oxidant gas in the presence of abed of supported oxidation catalyst with a second aqueous alkali metalhydroxide solution at a temperature and pressure and for a period oftime sufficient to oxidize at least a portion of the remaining mercaptancompounds from said treated hydrocarbon distillate; and recovering theproduct hydrocarbon distillate from such catalyst zone.
 7. The improvedprocess of claim 6, further comprising the step of feeding saidhydrocarbon distillate, gaseous oxygen oxidant, first aqueous alkalimetal hydroxide solution and oxidation catalyst into a vessel havingsaid fiber bundle, said bed of supported oxidation catalyst and aseparator therein.
 8. The improved process of claim 6, wherein the stepof contacting the hydrocarbon distillate in the presence of said fiberbundle comprises the steps of:introducing said first aqueous alkalimetal hydroxide solution having said oxidation catalyst therein onto anupstream end of said fiber bundle; and flowing said hydrocarbondistillate having said gaseous oxygen oxidant dissolved thereinconcurrently with and in contact with said first alkali metal hydroxidesolution.
 9. The improved process of claim 6, wherein the step ofseparating said first alkali metal hydroxide solution from said treatedhydrocarbon distillate comprises the steps of:discharging said firstaqueous alkali metal hydroxide solution and said treated hydrocarbondistillate from said fiber bundle into a separator; collecting saidfirst aqueous alkali metal hydroxide solution in the lower portion ofsaid separator; and collecting said treated hydrocarbon distillate inthe upper portion of said separator.
 10. The improved process of claim6, wherein the step of contacting said treated hydrocarbon distillateand oxidant gas in the presence of said bed of supported oxidationcatalyst comprises the steps of:passing said treated hydrocarbondistillate and oxidant gas from said separator through said bed ofsupported oxidation catalyst; introducing said second stream of aqueousalkali metal hydroxide solution into said bed of supported oxidationcatalyst; and flowing said second aqueous alkali metal hydroxidesolution counterconcurrently through said bed of supported oxidationcatalyst with and in contact with said treated hydrocarbon distillate.